1. Field of the Invention
The present invention relates to a semiconductor laser device having an internal current confinement structure, and a process for producing a semiconductor light emitting device having an internal current confinement structure.
2. Description of the Related Art
(1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 xcexcm band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that the semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.102 discloses a semiconductor laser device which emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.48Ga0.52As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, an Al0.2Ga0.8As/In0.2Ga0.8As double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al0.67Ga0.33As etching stop layer, a p-type Al0.48Ga0.52As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al0.67Ga0.33As etching stop layer by normal photolithography and selective etching, and an n-type Al0.7Ga0.3As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.
However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.
(2) In addition, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp.1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.4Ga0.6As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type Al0.4Ga0.5As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al0.4Ga0.6As upper cladding layer by normal photolithography and selective etching, and an n-type In0.5Ga0.5P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.
However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having a different V-group component, on such an upper cladding layer.
(3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.189 discloses an all-layer-Aluminum-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by normal photolithography and selective etching, and an n-type In0.5Ga0.5P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.
The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.
(4) Furthermore, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp.1889 discloses an internal striped structure semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.8 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type AlGaAs lower cladding layer, an AlGaAs/GaAs triple quantum well active layer, a p-type AlGaAs first upper cladding layer, an n-type AlGaAs current confinement layer, and an n-type AlGaAs protection layer are formed in this order. Next, a narrow-stripe groove is formed, by normal photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Next, over the above structure, a p-type AlGaAs second upper cladding layer and a p-type GaAs contact layer are formed.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in a fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type AlGaAs second upper cladding layer. However, the above semiconductor laser device also has a drawback that it is difficult to form an AlGaAs layer on another AlGaAs layer since the AlGaAs layers are prone to oxidation.
As described above, in the conventional current semiconductor laser devices which include an internal current confinement structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 xcexcm band, it is difficult to form an upper layer on a current confinement layer when aluminum exists near the boundary of the current confinement layer and the upper layer, since the AlGaAs layers are prone to oxidation. Even if the upper layer can be formed, defects occur at the boundary of the current confinement layer and the upper layer for the same reason.
An object of the present invention is to provide a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.
Another object of the present invention is to provide a process for producing a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, and formed on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 compressive strain quantum well active layer; a first upper cladding layer made of In0.49Ga0.51P of a second conductive type, and formed on the upper optical waveguide layer; an etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, and formed on the first upper cladding layer other than a stripe area of the first upper cladding layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.3, 0xe2x89xa6y1xe2x89xa60.6, and an absolute value of a second product of a strain and a thickness of the etching stop layer is equal to or smaller than 0.25 nm; a current confinement layer made of In0.49(Alz1Ga1xe2x88x92z1)0.51P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove, where 0xe2x89xa6z1xe2x89xa60.1; a cap layer made of In0.49Ga0.51P, and formed on the current confinement layer so as to form a third portion of the stripe groove; a second upper cladding layer made of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 of the second conductive type, and formed on the current confinement layer and the stripe area of the first upper cladding layer, where x4=(0.49xc2x10.01)y4 and 0.9xe2x89xa6y4xe2x89xa61; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the cap layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
The first conductive type is different in polarity of carriers from the second conductive type. That is, when the first conductive type is n type, and the second conductive type is p type.
The strain of the compressive strain quantum well active layer is defined as (caxe2x88x92cs)/cs, where cs and ca are the lattice constants of the GaAs substrate and the compressive strain quantum well active layer, respectively.
The strain of the etching stop layer is defined as (csxe2x88x92cs)/cs, where cs and cs are the lattice constants of the GaAs substrate and the etching stop layer, respectively.
When a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the substrate. That is, in the semiconductor laser devices according to the first and second aspects of the present invention, the absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003 for each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, and the second upper cladding layer.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (vi).
(i) The semiconductor laser device may further include first and second tensile strain barrier layers both made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and the absolute value of a sum of the first product and a third product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm. The strain of the first and second tensile strain barrier layers is defined as (cbxe2x88x92cs)/cs, where cb is the lattice constant of the first and second tensile strain barrier layers.
(ii) The etching stop layer may be one of the first and second conductive types.
(iii) The first cap layer may be one of the first and second conductive types and an undoped type.
(iv) The lower optical waveguide active layer may be the first conductive type, and the upper optical waveguide active layer is the second conductive type.
(v) The compressive strain quantum well active layer may have a multiple quantum well structure.
(vi) The stripe groove may have a width equal to or greater than 1 xcexcm.
(2) According to the second aspect of the present invention, there is provided a process for producing a semiconductor laser device, including the steps of: (a) forming a lower cladding layer of a first conductive type on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 compressive strain quantum well active layer; (e) forming a first upper cladding layer made of In0.49Ga0.51P of a second conductive type, on the upper optical waveguide layer; (f) forming an etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, on the first upper cladding layer, where 0xe2x89xa6x1xe2x89xa60.3 and 0xe2x89xa6y1xe2x89xa60.6, and the absolute value of a second product of the strain and the thickness of the etching stop layer is equal to or smaller than 0.25 nm; (g) forming a current confinement layer made of In0.49(Alz1Ga1xe2x88x92z1)0.51P of the first conductive type, on the etching stop layer, where 0xe2x89xa6z1xe2x89xa60.1; (h) forming a first cap layer made of In0.49Ga0.51P, on the current confinement layer; (i) removing a stripe area of the first cap layer and a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (j) removing a stripe area of the etching stop layer so as to form a second portion of the stripe groove; (k) forming a second upper cladding layer made of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where x4=(0.49xc2x10.01)y4 and 0.9xe2x89xa6y4xe2x89xa61; and (l) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the first cap layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
That is, the semiconductor laser device according to the first aspect of the present invention can be produced by the process according to the second aspect of the present invention.
Preferably, the process according to the second aspect of the present invention may also have one or any possible combination of the following additional features (vii) to (xi).
(vii) The process may further include, after the step (h), the steps of, (hi) forming a second cap layer made of GaAs, and (h2) removing a stripe area of the second cap layer. In addition, in the step (j), a remaining area of the second cap layer is removed together with the stripe area of the etching stop layer so as to form an additional portion of the stripe groove.
(viii) The etching stop layer may be one of the first and second conductive types.
(ix) The first cap layer may be one of the first and second conductive types and an undoped type.
(x) The second cap layer may be one of the first and second conductive types and an undoped type.
(xi) The process may further include the steps of, (b1) after the step (b), forming a first tensile strain barrier layer made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, on the lower optical waveguide layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and (c1) after the step (c), forming a second tensile strain barrier layer made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, on the compressive strain quantum well active layer, where an absolute value of a sum of the first product and a third product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(3) The first and second aspects of the present invention have the following advantages.
(a) When aluminum exists near a boundary surface on which the second upper cladding layer is formed, the boundary surface is prone to oxidation. However, in step (k) of the process according to the second aspect of the present invention, aluminum exists in only the In0.49(Alz1Ga1xe2x88x92z1)0.51P current confinement layer, which appears in only the side surfaces of the stripe groove. In addition, the aluminum content in the In0.49(Alz1Ga1xe2x88x92z1)0.51P current confinement layer does not exceed 10%. Therefore, it is easy to form the second upper cladding layer, and the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved.
(b) In the semiconductor laser device according to the first aspect of the present invention, the current confinement layer is made of In0.49(Alz1Ga1xe2x88x92z1)0.51P, and the second upper cladding layer is made of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4. Therefore, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes a difference of about 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923 in the equivalent refractive index of the active layer between the portion under the current confinement layer and the portion under the stripe groove, with high accuracy, and it is possible to cut off oscillation in higher modes. Thus, oscillation in the fundamental transverse mode can be maintained even when the output power becomes high.
(c) The etching stop layer is made of InGaP. Therefore, when a hydrochloric acid etchant is used, InGaAsP layers under the etching stop layer are not etched. That is, when a hydrochloric acid etchant is used, it is possible to accurately stop etching at the lower boundary of the first upper cladding layer. Thus, the stripe width can be accurately controlled, and the index-guided structure can be built in with high accuracy.
(d) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.
(e) When the tensile strain barrier layers are provided as described in the paragraphs (1)(i), various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.
(f) When the GaAs second cap layer is used as described in paragraph (2)(vii), it is possible to prevent formation of a natural oxidation film on the InGaP first cap layer, and metamorphic change in the InGaP first cap layer, which may occur when a resist layer is formed directly on the InGaP first cap layer. In addition, since the GaAs second cap layer is removed before the second upper cladding layer is formed, it is possible to remove a residue left on a boundary surface on which the second upper cladding layer is formed, and prevent occurrence of crystal defects.